One critical aspect of many optical elements is the curvature of the surface of the element. Indeed, refractive lenses derive their ability to converge or diverge light rays from the difference in curvature of their front and rear surfaces. Similarly, the focal plane of curved mirrors is determined by the mirror curvature. For most lenses and mirror applications, the surfaces in question have a spherical shape. One exception to this is lenses for correcting astigmatism which have a curvature that is a combination of spherical and cylindrical surfaces. The key feature of all these elements is that they have two-dimensional surfaces and, therefore, they have two principle curvatures. For example, a flat surface has both curvatures equal to zero. A cylindrical surface has one curvature zero while the other curvature is non-zero. A sphere may be defined as having both curvatures equal to one another and are non-zero. Accordingly, it will be appreciated that optical elements, with almost no exceptions, are constructed using surfaces for which both curvatures are non-zero and non-equal. Accordingly, those skilled in the art refer to these types of elements as doubly curved.
The most common optical element is the vision-correction lens used in spectacles. For all but the most severe prescriptions, these lenses are meniscus lenses, in which both surfaces are doubly curved. Corrective lenses may be fabricated having one surface doubly curved and the other flat, but this construction is undesirable for esthetic reasons, inasmuch as human faces are also doubly curved. In addition to vision correction, spectacles with doubly curved lenses are worn to protect the eyes from sunlight, glare, and foreign objects, and of course, they are also used as fashion accessories. Other types of eyewear having doubly curved surfaces are goggles, visors, and helmet face plates. Other examples of doubly curved surfaces which light must pass either through or reflect from are windshields, glass block windows, automobile headlamps, skylights, and related optical devices.
For these and many other applications, it is common to affix a solid layer or layers unto the surface of an optical element. The layer or layers usually are affixed to provide additional optical functionality, such as light transmission control, or anti-reflective properties. Accordingly, each additional layer acts as an optical element in its own right and when it is attached to another element, the result is a compound element. Various difficulties arise when attempting to manufacture optical elements and one of these layers is attached to a doubly curved surface. In particular, the layer to be affixed to the doubly curved surface is initially flat. For example, one may create a pair of “mirrored sunglasses” by affixing aluminized Mylar® onto the lenses of an ordinary pair of glasses. It is quickly seen that unless the initially flat Mylar® is either stretched or cut, it cannot be conformally attached to the doubly curved lens surface. Alternatively, the initially flat layer may be affixed by changing the state of the layer material during the affixing process. If the layer is softened, or even melted and affixed to the state, it can be conformally attached. Obviously, the resulting compound optical element must then be operated at a temperature lower than the temperature at which the layer was affixed.
Although affixing solid layers in the manner described above has been accomplished, many more difficulties arise when it is desired to affix multiple layers to an optical element, especially when these two layers are separated from one another by a controlled distance. In other words, this controlled distance provides a gap between the two optical layers and this gap, extending over the area of the optical element, creates an encapsulated volume. This encapsulated volume may be occupied by a fluid substance or substances that perform desired optical, protective, or other functions. One such device is disclosed in U.S. Pat. No. 6,239,778, which is incorporated herein by reference. The resulting compound optical element would then have electronically controllable light transmission. Those skilled in the art will appreciate that maintaining the gap in such devices is critical to ensure correct operation.
Attempts at providing a controlled gap between two doubly curved surfaces with a fluid material therebetween has been found to be quite problematic. One attempt at solving this problem is to employ doubly curved half-lenses which are separated by spacers of the desired gap distance. However, due to the small cell gaps that are required for such devices—on the order of microns—it is difficult to properly align both lenses while maintaining the required gap distance over the entire area of the lenses. It will be appreciated if the proper gap spacing is not maintained; the desired optical properties are likewise unattainable. And it is has been found to be quite difficult to properly shape the outer surfaces of such devices so that they conform to the shape of adjacent optical elements.
Other methods for obtaining ophthalmic lenses can be tailored to correcting individual's vision by cementing together two stock “half-lenses” such that the resulting compound lens has the correct prescription. This method of producing of laminated lenses is described in U.S. Pat. Nos. 4,883,548 and 6,180,033. However, these disclosures do not address the need for maintaining a controlled gap distance between the lenses so that a fluid material can be employed to control light transmission or reflection properties. Accordingly, there is a need in the art for a device and a method for making the same that provides curved surfaces in which a gap can be repeatably maintained between the curved surfaces.
Although the methodology developed for forming a curved optical device having a gap between opposed substrates has been found effective, it will be appreciated that different end-use applications for optical devices may require different thermoforming techniques. In other words, the best thermoforming technique depends on many factors including the choice of substrate material or area or curvature of the optical device. Therefore, there is a need for methodologies to form an optical device that is adaptable to the different types of substrates or other features of an optical cell that are required for an end-use application. In particular, substrate material size, curvature, optical clarity and other requirements of a curved optical device require advancements in the techniques needed for forming the same.